Local hyperthermia is used adjunctively with radiation therapy (RT), the standard therapy for most lung cancer (LC), to increase the efficacy of RT for other cancers. Localized unresectable tumors account for 40% of LC's, yet no hyperthermia modality effectively provides localized deep heating in the lung. An new approach and system is proposed for LC hyperthermia based on propagating focused ultrasound (US) through lung tissue which has been temporarily filled with non-toxic liquid(s) of unique biological and lung-function compatible properties. The liquids, termed "per fluorocarbons" (PFC), provide the acoustic propagation medium. The procedure will be done in a manner which continually sustains patient respiration. Fortunately,the most important subsidiary technologies, clinical practices and biological knowledge needed to realize this treatment are already very advanced. Consequently, feasibility rests mainly upon acoustic considerations and less upon medical ones. In addition to improving LC therapy, the techniques envisioned have a strong potential to benefit LC diagnostic procedures and have many other ancillary advantages. The Phase I research objectives are to thoroughly characterize the acoustics behavior of these liquids in vitro and in isolated animal lungs, and then to perform a short series of experimental hyperthermia treatments on animals (sheep) using this technique.